I. Field of the Invention
The present invention is directed generally to mobile telephone systems. More particularly, the present invention relates to systems and methods that permit a mobile telephone user to roam outside of the user's home service area. Still more particularly, the present invention relates to systems and methods for storing and quickly accessing records in a Preferred Roaming List (PRL) from memory in a mobile telephone.
II. Description of the Related Art
In typical mobile telephone systems, when a mobile telephone user moves outside the user's home area (also known as roaming), the user's telephone is often switched from the home system to an alternate system provider so that service can continue for the user. In order to switch the user to an alternate system provider, the mobile telephone will typically access system records in the PRL stored in the memory of the mobile phone. Based on the system records stored in the PRL, the mobile phone will typically select an alternative system service provider that will be used to continue service for the user and, based on this selection and the associated information stored in the PRL system records, the mobile phone will automatically access the selected service provider on behalf of the user.
The Telephone Industry Association/Electronics Industry Association (also known as TIA/EIA) has recently published a new standard that describes a data format to be used for system table records in a PRL. See Interim Standard entitled "Over-The-Air Service Programming Of Mobile Stations in Spread Spectrum Systems", (hereinafter "the IS-683-A Standard"), the contents of which is hereby incorporated by reference in its entirety. The IS-683-A Standard provides that each system record in the PRL will have the following eight fields:
1. SID field: a 15 bit field that includes a numerical value that is the System Identification number of the system associated with the system record; PA1 2. NID_INCL field: a 2 bit field, the least significant bit of this field indicates whether there is a Network Identification (NID) number associated with the system record and stored in the NID field described below; the most significant bit of this record is used to specify an assumed value for the NID if no value is stored in the NID field described below; the value 11 is a reserved value; PA1 3. NID field a 16 bit field that contains a numerical value that is the Network Identification number of the system associated with the system record; PA1 4. PREF_NEG field: a 1 bit field indicating whether the system associated with the system record is a preferred system or a negative system. PA1 5. GEO field: a 1 bit field that indicates whether the system associated with the system record is within the geographic region of the previous system record in the PRL (if GEO=1) or, alternatively, whether the system is in a new geographic region (if GEO=0); PA1 6. PRI field: a 1 bit field that indicates whether the current system record has a higher priority than that of the next system record in the PRL. This field is omitted from the system record if the PREF_NEG field indicates that the system associated with system record is a negative system; PA1 7. ACQ_INDEX field: a 9 bit field that contains an index to the acquisition parameters required for the mobile phone to access the system associated with the system record; PA1 8. ROAM_IND field: an 8 bit field that indicates the roaming status of the associated system record; this field is omitted from the system record if the PREF_NEG field indicates that the system associated with system record is a negative system.
In current mobile telephones, the PRL is stored in the mobile phone's non-volatile memory and, during system initialization, the entire PRL is moved and stored in the unit's RAM in a single bit-packed array for later access during a System Selection and Preferred Roaming operation. One purpose of using the bit-packed form is to save memory space. Unfortunately, since each record in the PRL is variably-sized (this occurs because, as described above, some or all of the NID, PRI and ROAM_IND fields can be omitted from a given system record), the starting position in RAM of each system record in the PRL is uncertain. This uncertainty makes accessing a given system record difficult and time-consuming in current systems. More particularly, as a result of the uncertainty of the starting position in RAM of each system record in the PRL, current record accessing software must sequentially step through each system record in the System Table stored in RAM starting with the first record in the table in order to access a record that is located somewhere within the table. One other deficiency of current record accessing systems is that they do not permit the "backtracking" of records within the PRL stored in RAM. For example, if the system previously accessed the 164th record in the PRL stored in RAM and, thereafter, the system wished to access the 161st record, the accessing software would be unable to simply go back three records in the table and would instead be forced to again sequentially step through each system record in the System Table stored in RAM starting with the first record in the table in order to access the 161st record.
These problems and deficiencies are recognized and solved by the present invention in the manner described below.